The present invention relates generally to an orthopedic medical device implant. In particular, the present invention is related to a unicondylar knee implant system's tibial component.
Orthopedic knee implant systems have been used for many years to treat patients with knee joints that have been damaged by trauma or disease, such as osteoarthritis, rheumatoid arthritis, and avascular neurosis. A knee arthroplasty resects, cuts, or resurfaces the damaged sections of the knee and replaces them with an endoprosthetic or implant.
Most knee implant systems are tricompartmental implants and the surgical procedure used with tricompartmental implants is commonly known as total knee arthroplasty. These implants are known as tricompartmental implants because they are used when the knee joint is prepared to receive an implant by resurfacing or resecting the three articulating compartments, i.e., the medial and lateral femorotibial and the patellofemoral surfaces. Regardless of the type of implant used, all arthroplasties require the bone to be specifically prepared to receive a corresponding implant by resecting, resurfacing, or deforming the bone to accept the implant.
Unicondylar or unicompartmental knee implants have become of great interest in the orthopedic industry due to their less invasive nature and the maintaining of the other healthy knee compartments. Unicondylar knees resurface or resect typically the medial or lateral femorotibial articulating surfaces thus allowing preservation of the other compartments which may not be suffering from damage due to trauma or disease.
Generally, the clinical outcomes for unicondylar knee implants have varied. Studies have reported long term survival rates for unicondylar implants to be less than that of comparable total knee implants. One particular cause for such discrepancies is due to the bone cement fixation technique associated with the tibial implant. Another cause is the limitations on longer term cement fixation integrity. And, another cause is the non-physiological tibial bone loading patterns of a required metal backed tibial component that is relatively stiff compared to the surrounding bone.
The development of orthopedic implant designs has been moving towards meeting the requirements of high demand patients. Patients today are requiring more from their implants and since patients are living longer, they are requiring implants to last longer. Accordingly, developments have been made in materials used to make orthopedic implants to improve implant survival rates, such as highly porous metals for biological bone fixation.
Orthopedic devices are mated with host bone by either cementing them in place using methyl methacrylate, generally termed bone cement, or by providing a rough or porous surface on the device for bone tissue to grow into, generally termed press-fit or cementless.
The use of bone cement in attaching a prosthesis within or onto a prepared bone provides an excellent immediate fixation but has various disadvantages that appear over time. Physical loads are repeatedly applied to the implant over its life. If bone cement is used to secure a unicompartmental knee prosthesis, the bone cement may fatigue and fracture under the repeated loading. In some instances, degradation of the bone cement integrity may cause the device to become loose, thereby necessitating replacement. Old bone cement must be removed from the host bone as part of the implant replacement procedure. This procedure can be complex, time consuming and potentially destructive to healthy bone structures surrounding the implant. Furthermore, conventional bone cement is cured after it has been dispensed into the patient's joint. Loose undetected cement fragments can remain in the joint space and, with patient mobility over time, increase the degradation rate of articulating implant surfaces.
Recognizing the disadvantages of cement fixation techniques, prior art devices have been developed that utilize mechanical attachment means to join an implant to bone for immediate stabilization. Various implant surface treatments intended to bond with bone biologically for long term stable attachment have proven successful. A simple technique of mechanically securing an implant, is to affix it within the bone with screws or other mechanical fasteners. However, due to the nature of the bone surrounding the surgical site, and other limiting factors such as artery location and the like, screws can only be applied in certain limited regions. The use of a screw for implant fixation should be considered only as an option by the surgeon depending upon implant placement and bone quality.
Primary fixation of an implant should come from a high friction interface with the prepared bone and in the long term with bone tissue ingrowth into a porous portion of the device. Specific instruments and surgical procedures are developed to match the implant and bone preparation. Often the bone cuts are undersized so that the implant or a portion of the implant such as a peg or keel is “press fit” into the bone. This assures an intimate contact between bone and implant. A high friction coating or porous portion of the implant assists with immediate bone fixation by mechanically locking the device in place. High friction will also resist any loading which may displace the device prior to bone ingrowth and more permanent biological fixation.
Prior art has established many methods for producing a high friction porous layer for implant designs. The use of metal beads, particles or wires which are metalurgically bonded to the implant surface is common. Plasma coating of metal surfaces with rough layers of metal particles is also utilized. More recently, porous metals of various chemical make up and structure have been developed which mimic the design of bone trabecular structure. These materials have been shown to have superior bone ingrowth results and should lead to improved implant fixation.